Conventional swimming pool installations comprise a filter and a pump. The vast majority of filters currently in use are sand filters. However, some diatomaceous earth and other filters are in use. A sand filter conventionally comprises a casing having a water inlet at the top and a water outlet at the bottom. The pump withdraws water from a skimmer box forming part of the pool, forces it through a pressure pipe leading to the top of the filter, through the sand in the filter from the top to the bottom and then feeds the water emerging from the filter outlet back to the pool along a return pipe.
When the filter becomes blocked, two valves of the installation are closed and two others are opened. This is usually done manually and has the effect of directing the water emerging from the pump to the bottom of the filter instead of the top. Water flowing upwardly through the sand in the filter in the opposite direction to normal agitates the sand and dislodges dirt particles. The dirt is carried away by the water to a waste pipe.
It is known to interconnect the valves so that they operate in unison. Thus a valve in the waste pipe opens and closes simultaneously with a valve in a pipe joining said pressure and return pipes, and valves in the pressure and return pipes also open and close simultaneously to enable backwashing to take place.
All filters block over a period of time and hence all have to be backwashed to clean them and restore full flow.
Other bodies of water confined in tanks exhibit similar problems to those encountered with swimming pools. For example, tanks in which shellfish such as abalone are cultivated foul and have to be cleaned in the same way that a swimming pool does.